Autonomic nervous system (ANS) activity provides basic physiological responses to many activities of a person. For example, instantaneous autonomically mediated responses such as environmental mechanical stimulus may help to prevent a loss of blood pressure to the upper extremities of the person due to a sudden posture change like rising from a supine position. This physiological response is achieved by peripheral vasoconstriction of the lower extremities modulated by the ANS. Decreased function of the ANS can lead to undesirable symptoms and ailments, such as loss of consciousness upon a sudden posture change. In addition, decreased ANS function is associated with a significantly worse prognosis for heart failure patients.
ANS activity, or autonomic tone, further influences sino-atrial (SA) node and atrioventricular (AV) node function. An increase in sympathetic tone can result in an increased heart rate and an increased rate of AV nodal conduction. Conversely, an increase in parasympathetic tone can result in decreased heart rate and an increase in AV nodal conduction time. Due to these heart-related indications of autonomic tone, heart rate and heart rate variability are two diagnostic tools commonly used to obtain information about the state of the ANS. Indeed, decreases in heart rate variability, as well as increases in night heart rate, are known to be associated with an increasing incidence of heart failure. Implantable medical devices, therefore, have been used to measure heart rate and heart rate variability for providing indices of autonomic tone to a clinician.